R1:682e0aa468f6

Add "responsible users" and "subscribers" to DifferentialRevisionQuery, plus a
couple bug fixes

Summary:
- Add the ability to query for "responsible users" (author or reviewer).
- Add the ability to query for "subscribers" (reviewer or CC).
- Fix an issue where CC and Reviewer used the same join table alias and were
incompatible.
- Remove support for 'paths' for the moment, since each path needs a
repository ID. (There are no clients for this.)
- Remove single withX() methods that have no…